TEHRAN, Iran (AP) — Iran on Friday demanded the British navy release an Iranian oil tanker seized last week off Gibraltar, accusing London of playing a “dangerous game” and threatening retribution, while London announced it was sending a destroyer to the Persian Gulf.

The comments from Iran’s Foreign Ministry came the day after police in Gibraltar, a British overseas territory on the southern tip of Spain, said they arrested the captain and chief officer of the supertanker suspected of breaching European Union sanctions by carrying a shipment of Iranian crude oil to Syria.

Foreign Ministry spokesman Abbas Mousavi told Iranian state news agency IRNA that “the legal pretexts for the capture are not valid … the release of the tanker is in all countries’ interest.”

“Rest assured, Britons will soon feel the slap of the powerful hands of the Islamic Republic,” he said.

The British navy said Thursday it had stopped three Iranian paramilitary vessels from disrupting the passage of a British oil tanker through the Strait of Hormuz, a critical shipping lane at the mouth of the Persian Gulf. Iran’s Revolutionary Guard denied any incident had occurred in the strait.

That brief but tense standoff is believed to have stemmed from the British seizure of the Iranian tanker off Gibraltar on July 4.

On Friday, the British Ministry of Defense said it was moving up its timetable for relieve the HMS Montrose, a frigate operating in the Persian Gulf, with the larger HMS Duncan destroyer in the wake of the recent developments.

“This will ensure that the UK alongside international partners can continue to support freedom of navigation for vessels transiting through this vital shipping lane,” the ministry said.

At the same time, British Foreign Secretary Jeremy Hunt called for “cool heads” to prevail to ensure there is no “unintended escalation.”

“We are being clear to Iran that we are not seeking to escalate this situation,” he said, adding that the British response had been “measured and careful.”

The Iranian tanker intercepted last week was carrying 2.1 million barrels of light crude oil, the head of Gibraltar’s government said Friday.

A senior Spanish official had previously said the interception was carried out at the request of the United States, but Gibraltar’s Chief Minister Fabian Picardo told parliament no other government had asked the territory to act.

“These important decisions about breaches of our laws were certainly not decisions taken at the political behest or instruction of any other state or of any third party,” he said.

He said the ship is suspected of breaching European Union sanctions on Syrian President Bashar Assad’s government and that any nation with a claim to the vessel and its cargo can file a claim in court.

In addition to the tanker’s captain and chief officer arrested Thursday, Gibraltar police said they arrested the two second mates of the Panama-flagged tanker on Friday. The two are in custody and are assisting police with their inquiries, authorities said. All four are Indian citizens.

Iranian Foreign Minister Mohammad Javad Zarif scoffed at the accusation Iran was violating sanctions, which he said “are meant to stop Europe from buying Syria’s oil, they are not about another country selling oil to Syria.” Zarif spoke in an interview Thursday with the pro-Iran Lebanese satellite news channel Al-Mayadeen.

“This is a very childish and ridiculous excuse by the British,” he said “They should officially announce that we are servants of America and act on behalf of America. America has returned their favor well by insulting their ambassador and their prime minister.”

The tanker’s interception has stoked already high tensions in the region, as the Trump administration continues its campaign of maximum pressure on Iran.

The U.S. has sent thousands of troops, an aircraft carrier, nuclear-capable B-52 bombers and advanced fighter jets to the Middle East, and fears are growing of a wider conflict after mysterious oil tanker attacks near the Strait of Hormuz blamed on Iran, attacks by Iranian-backed rebels in Yemen on Saudi Arabia and Iran’s downing of a U.S. military drone.

Iran has recently begun surpassing uranium enrichment limits set in its 2015 nuclear deal with world powers in response to President Donald Trump’s decision to pull the U.S. out of the accord a year ago.

The U.S. has also re-imposed tough sanctions on Tehran’s oil exports, exacerbating an economic crisis that has sent its currency plummeting.

Iran has said its breaches of the nuclear pact can be reversed if the other parties to the agreement — Germany, France, Britain, China, Russia and the European Union — can come up with enough economic incentives to effectively offset the American sanctions.

China on Friday reiterated it opposes unilateral sanctions against Iran and criticized what it described as the “long arm jurisdiction” of the United States.

A Foreign Ministry spokesman in Beijing said that international trade with Iran “within the framework of international law is reasonable and legitimate and deserves to be respected and protected.”